The present invention relates generally to stranded cables and their method of manufacture. In particular, the invention relates to stranded cables comprising helically wound brittle wires and their method of manufacture. Such stranded cables are useful in power transmission cables and other applications.
Cable stranding is a process in which individual wires are combined, typically in a helical arrangement, to produce a finished cable. See, e.g., U.S. Patent Nos. 5,171,942 and 5,554,826. The resulting stranded cable or wire rope provides far greater flexibility than would be available from a solid rod of equivalent cross sectional area. The stranded arrangement is also beneficial because the stranded cable maintains its overall round cross-sectional shape when the cable is subject to bending in handling, installation and use. Such stranded cables are used in a variety of applications such as hoist cables, aircraft cables, and power transmission cables.
Such helically stranded cables are typically produced from metals such as steel, aluminum, or copper. In some cases, such as bare overhead power transmission cables, the helically stranded core could comprise a first material such as steel, for example, and the outer power conducting portion could comprise another material such as aluminum, for example. In this case, the core may be a pre-stranded cable used as a input material to the manufacture of the larger diameter power transmission cable.
Helically stranded cables may comprise as few as 7 individual wires to more common constructions containing 50 or more wires. Prior to being helically wound together, the individual wires are provided on separate bobbins which are then placed in a number of motor driven carriages of the stranding equipment. Typically, there is one carriage for each layer of the finished stranded cable. The wires of each layer are brought together at the exit of each carriage and arranged over the central wire or over the preceding layer. During the cable stranding process, the central wire, or the intermediate unfinished stranded cable which will have one or more additional layers wound about it, is pulled through the center of the various carriages, with each carriage adding one layer to the stranded cable. The individual wires to be added as one layer are simultaneously pulled from their respective bobbins while being rotated about the central axis of the cable by the motor driven carriage. This is done in sequence for each desired layer. The result is a helically stranded cable which can be cut and handled conveniently without loss of shape or unraveling. This attribute may be taken for granted but is an extremely important feature. The cable maintains its helically stranded arrangement because during manufacture, the metallic wires are subjected to stresses beyond the yield stress of the wire material but below the ultimate or failure stress. This stress is imparted as the wire is helically wound about the relatively small radius of the preceding layer or central wire. Additional stresses are imparted at the closing die which applies radial and shear forces to the cable during manufacture. The wires therefore plastically deform and maintain their helically stranded shape.
There have been recently introduced useful cable articles from materials that are brittle and thus cannot readily be plastically deformed to a new shape. Common examples of these materials include fiber reinforced composites which are attractive due to their improved mechanical properties relative to metals but are primarily elastic in their stress strain response. Composite cables containing fiber reinforced polymer wires are known in the art, as are composite cables containing ceramic fiber reinforced metal wires, see, e.g., WO 97/00976.
In the case of fiber reinforced polymer matrix wires, the individual wires in the cable can be thermally set after stranding to maintain a helical arrangement. In such an arrangement, the helically wound cables do not need some means to maintain the helical arrangement. For example, U.S. Pat. No. 5,126,167 describes a process for the manufacture of a fiber reinforced plastic armored cable. In this process, long reinforcing fibers are impregnated with an uncured thermosetting resin and formed into a predetermined shape to obtain a plurality of rod-like members with the thermosetting resin held uncured. Then the uncured rod-like members are passed through a die of a melt extruder, by which the rod-like members are each coated with a thermoplastic resin layer. The coated layers of the rod-like members are immediately cooled to simultaneously form a plurality of fiber reinforced plastic armoring wires with the thermosetting resin held uncured. The armoring wires thus obtained are wound around a cable which is fed while being rotated. The cable having wound thereon the wires is passed through a die portion of a melt extruder, by which the cable is sheathed with a thermoplastic resin layer that is immediately cooled and solidified. The sheathed cable is guided into a curing tank using a liquid as a heating medium to cure the thermosetting resin in the armoring wires.
Tapes are wrapped around stranded cables for various reasons: as electrical shielding, as protection from the environment such as water or moisture, as an electrically insulating material particularly in underground or insulated overhead conductors, as a protective armor layer, or as a thermally insulating layer for high temperature applications. Japanese Patent Application HEI 3-12606 teaches an aerial power cable that has fiber reinforced plastics (xe2x80x9cFRPxe2x80x9d) as the core strength member. The background of the ""606 application says that fiber reinforced plastic cables have been previously suggested as a strength member for aerial power cables for increasing current and reducing sag but has the shortcomings that the fiber reinforced plastic has low heat resistance and low bend and impact resistance. The patent seeks to overcome these limitations by wrapping a fiber Is reinforced plastic wire with a metal tape or a heat resistant coating. The ""606 application discloses an embodiment in which a metal casing made of a metal tape is formed around the FRP wire. The metal tape is reported to function as a buffer layer and to reduce brittleness of the FRP wire upon bending or under impact. The ""606 application reports that at the same time, thermal deterioration of the resin inside can be effectively prevented and an aluminum cable reinforced with FRP having long-term reliability can be produced. The ""606 application also proposes an embodiment to protect the individual fiber reinforced plastic wires by wrapping each plastic wire with a metal tape (shown in FIG. 4) or coating it with a heat resistant binder.
WO 97/00976 describes in one embodiment an arrangement of fiber reinforced composite wires that forms a core. The core is surrounded by a jacket of monolithic metal wires that serve as a conductor for a power transmission cable. See FIGS. 2a and 2b of the ""976 publication. The wires in the core comprise a metal matrix of polycrystalline xcex1-Al2O3 fibers encapsulated within a matrix of substantially pure elemental aluminum, or an alloy of elemental aluminum and up to about 2% copper. These wires are brittle and not susceptible to significant plastic deformation.
While many of the above approaches enjoy some degree of success, it is desirable to further improve the construction of the helically stranded core and its method of manufacture. For example, it is desirable to provide a helically stranded cable that includes brittle wires. It is desirable to provide a convenient means to maintain the helical arrangement of the brittle wires prior to incorporating the core into a subsequent article such as a power transmission cable. Such a means for maintaining the helical arrangement has not been necessary in prior cores with plastically deformable wires or with wires that can be cured or set after being arranged helically.
In one aspect, the present invention provides a stranded cable. The cable comprises a plurality of brittle wires in which the brittle wires are stranded about a common longitudinal axis. The brittle wires have a significant amount of elastic bend deformation. The cable also includes adhesive means for maintaining the elastic bend deformation of the wires. In one preferred embodiment, the maintaining means comprises an adhesive tape wrapped around the plurality of brittle wires. The adhesive tape may comprise a pressure sensitive adhesive. In another preferred embodiment, the maintaining means comprises a binder. The binder may comprise a pressure sensitive adhesive.
In another aspect, the present invention provides an alternative embodiment of a stranded cable. The stranded cable comprises a plurality of brittle wires stranded about a common longitudinal axis. The brittle wires have a significant amount of elastic bend deformation. The stranded cable also includes maintaining means for maintaining the elastic bend deformation of the wires, in which the outer diameter of the stranded cable including the maintaining means is no more than 110% of the outer diameter of the plurality of stranded brittle wires excluding the maintaining means. In one preferred embodiment, the maintaining means comprises a tape wrapped around the plurality of brittle wires. Preferably, the tape comprises an adhesive tape. In another preferred embodiment, the maintaining means comprises a binder adhered to the plurality of brittle wires. Preferably, the binder comprises a pressure sensitive adhesive.
In either or both of the above two embodiments of stranded cables, the following embodiments may be employed:
In one preferred embodiment, the brittle wires each comprise a composite of a plurality of continuous fibers in a matrix. The matrix preferably comprises a metal matrix. More preferably, the metal matrix comprises aluminum and the continuous fibers comprise polycrystalline xcex1-Al2O3.
In another preferred embodiment, the brittle wires are continuous and at least 150 m long. More preferably, the continuous brittle wires are at least 1000 m long.
In another preferred embodiment, the brittle wires have a diameter of from 1 mm to 4 mm.
In another preferred embodiment, the brittle wires are helically stranded to have a lay factor of from 10 to 150.
In another preferred embodiment, there are at least 3 stranded brittle wires. More preferably, the cable includes a central wire, and the stranded brittle wires are stranded in a layer about the central wire. Still more preferably there are at least two layers of the stranded brittle wires.
In another aspect, the present invention provides an electrical power transmission cable comprising a core and a conductor layer around the core, in which the core comprises any of the above-described stranded cables. In one preferred embodiment, the power transmission cable comprises at least two conductor layers. In another preferred embodiment, the conductor layer comprises a plurality of stranded conductor wires. In another preferred embodiment, the electrical transmission cable comprises an overhead electrical power transmission cable.
In still another aspect, the present invention provides another alternate embodiment of a stranded cable. The stranded cable comprises a plurality of brittle wires. The brittle wires are stranded about a common longitudinal axis and have a significant amount of elastic bend deformation. The stranded cable also includes a maintaining means for maintaining the elastic bend deformation of the wires. In this embodiment, the stranded cable is free of electrical power conductor layers around the plurality of brittle wires. Provided this embodiment is free of electrical power conductor layers around the plurality of brittle wires, any of the preferred embodiments described above may be employed with this embodiment.